1. Field of the Invention
This invention relates to the preparation of metal oxide powders, and more particularly to their preparation by spray calcination. In one of its more particular aspects, this invention relates to the preparation of fine grain multicomponent metal oxide powders of selected composition for conversion to their corresponding superconducting metal oxide ceramics.
2. Prior Art
Superconductivity is the property of certain materials at cryogenic temperatures approaching absolute zero to carry electric currents without power dissipation. Commercial superconductors such as niobium alloys do not reach the superconductive state until chilled below 23.degree. K. This requires the use of liquid helium, which condenses at 4.degree. K. However, more recently, new high-temperature superconductors showing a superconductive transition temperature well above 77.degree. K., allowing liquid nitrogen to be used as cryogenic fluid, have been reported by various laboratories. The high-temperature superconductive materials presently being studied consist of various metal oxides bonded together in a complex ceramic-like structure. For example, a typical laboratory recipe calls for heating an intimate mixture of the oxide or carbonate powders of the solid elements at temperatures between 900.degree. and 1100.degree. C. to drive off carbon dioxide and other volatiles. After regrinding and reheating, the mixture is pressed into pellets and sintered at high temperatures for several hours. The pellets are annealed in an oxygen environment at a lower temperature between 400.degree. and 500.degree. C. The resulting ceramics tend to be brittle and fragile and cannot be drawn out to form wires as can copper and other ductile metals.
Attempts have also be made to prepare thin superconducting films from multiphase Y-Ba-Cu-O material using evaporative techniques. Other workers have fabricated tapes that superconduct above 90.degree. K. by imbeddding ceramic particles in organic material.
Several laboratory approaches to the preparation of high-temperature superconductive materials have been reported. In Rev. Sci. Instrum. 58(9) September 1987, pages 1565-1571, Xiao-Dong et al have reported on the "Practical Preparation of Copper Oxide Superconductors" which involves use of either a solid state reaction method or a coprecipitation method. Each of these methods involve detailed laboratory steps preferably performed in a hood because of possible toxic problems involved.
Another approach involves the precipitation of superconductor precursor powders. A solution containing highly soluble metal salts is mixed with a solution containing highly soluble salts of precipitating anions. When the solutions are mixed, an insoluble precipitate is formed. The precipitate is heated to form the desired superconducting phases. This requires complex considerations relating to pH and anion control in the precipitation. Also, cation contamination is difficult to avoid and this generally interferes with the subsequent sintering and produces unwanted phases in the final ceramic.
Because of the laboratory-scale nature of the processes and the precise type of controls required when utilizing the mixed oxide or precipitation of oxides from solution, attempts have been made to adapt other techniques for the production of metal oxide powders. Thus, various single-component metal oxides are presently being produced by means of the thermal decomposition of the corresponding metal chloride solutions. Several spray roasting techniques have been proposed for the production of multicomponent ceramic powders (metal oxides). Since these techniques depend on the decomposition of metal chloride systems, incomplete transformations occur where the decomposition temperatures of the provided metal chlorides are too far apart.
Aerosol methods for the production of powders have been generally known. Recently, there has been described an experimental heated-wall apparatus, designated as an aerosol flow reactor, for the production of fine yttrium-barium-copper oxide powder for the fabrication of superconducting ceramics. Aerosol droplets of an aqueous solution of the Y, Ba and Cu nitrates in a 1:2:3 mole ratio are generated using two nebulizers, one capable of producing droplets with a mass median diameter of approximately 2 microns and the other of 0.5 micron. Pure oxygen is used as the carrier gas at a flow rate to provide an average residence time in the reactor of between 20 and 100 seconds. After the droplets are generated, the water vapor is removed by passing the droplets through a diffusion dryer. The dried particles are then passed through the reactor to convert the nitrates to the superconductive material. The powders are collected on high efficiency ceramic and glass fiber cartridge filters. The reported overall yield was approximately 50%, principally because of thermophoretic wall losses after the heated zone in the reactor.
In another process, aerosol droplets are formed from a water solution of dissolved metal nitrates and passed through a tubular furnace at temperatures of up to 1000.degree. C. The water evaporates, the nitrates decompose and oxygen occurs forming the metal oxide compound in the form of a fine powder. The residence time in this reactor varied from 15 to 56 seconds.
Spray calcination processes are known for the thermal conversion of wastes and the reduction of the volume of low-level radioactive wastes which contain free water. Spray converters and spray dryers are used for contacting the wastes, generally in slurry form. The sprayed particles are contacted in the spray converter with a hot gas, which results in the drying of the waste and the calcining and combusting of the dried waste. Such spray processes are particularly useful with wastes containing hazardous material such as radioactive substances. Such spray converters and spray dryers are shown in U.S. Pat. Nos. 4,499,833; 4,559,170; 4,579,069; and 4,668,435.
All of the foregoing processes, while of interest and feasible to some extent, are open to various technical and economic objections which militate against their widespread commercial adoption. In general, many processing steps are required which are frequently costly and require precise handling. Yields are generally low because of the relatively complex processing involved. Relatively long residence times (usually many seconds) can give volatile components of the mixture an opportunity to separate from the mixture. Hence, these processes are generally of laboratory interest only, lacking with respect to ease of operability and commercial feasibility.
Accordingly, it is an object of the present invention to provide a commercially useful process for the preparation of fine-grain multicomponent metal oxide powders of selected composition.
It is a further object to provide a process for producing high-purity, reproducible, intimately mixed, superconductor precursor powders in bulk quantities for conversion to high temperature superconductors.
Another object is to provide a process which is capable of controlling the emissions of hazardous, toxic materials and which is readily amenable to the production of a wide variety of different multicomponent superconductor precursor powders in bulk quantities on a cost-effective basis.
Other objects and advantages of this invention will become apparent in the course of the following detailed description.